In thermodynamics, a heat engine is a system that converts thermal energy into mechanical energy, which is then used to perform work. The system is able to carry out this energy conversion through changing a working fluid from a higher state (temperature and pressure) to a lower state (temperature and pressure). A heat source and pump increases the thermal energy in the working fluid, which usually results in a phase change from liquid to vapor. The working fluid then transfers its heat to a colder heat sink until the working fluid again reaches its lowest temperature state in the cycle. During this process, some of the thermal energy is converted into work by exploiting the properties and state change of the working fluid. The formula for the efficiency of this Rankine cycle is
      η    th    =                    W        net                    Q        H              =          1      -                        T          C                          T          H                    where TC is the absolute temperature of the cold reservoir, TH is the absolute temperature of the hot reservoir, and the efficiency ηth is the ratio of Wnet, the work done by the engine, to QH, the heat drawn out of the hot reservoir. Thus, the efficiency may be increased through lowering TC and QH or increasing TH and Wnet.
Non-hydro power generating plants, using a working fluid loop in the form of a Rankine cycle, draw power from a generator-turning turbine through which the working fluid flows in its vaporized state. The working fluid is vaporized from its liquid state at a boiler through various heat sources depending on the type of plant, such as burning fuel or nuclear thermal energy transfer. After leaving the turbine, the working fluid is returned to its liquid state through cooling and condensing at a condenser. The specific volume difference between the vapor phase and liquid phase, which is induced by the heat exchange with the cooling fluid at the condenser, helps to pull the vapor through the turbine.
For example, when water is the working fluid, the steam condenser is a heat exchanger located in the power plant steam system for condensing steam. The turbine exhaust steam enters the steam condenser, flowing around tubes with a coolant flowing through the tubes, thereby condensing the steam. In general, the colder the coolant, the greater the amount of condensing. Condensers are designed to operate under the worst case scenario when the coolant temperature is at its highest. This entails the base operating conditions for a working fluid loop.
Many current power plant designs utilize a coolant for the working fluid that may be subject to seasonal temperature changes. Sometimes, in colder months, the larger temperature difference between the coolant and the working fluid can increase the efficiency of the cycle at the condenser. However, this is not always possible. Instead, these potential energy savings are lost due to the particular limitations of the system components, such as the sonic velocity of the working fluid, for example.
In view of the aforementioned problems, the present disclosure provides systems and methods for constructing new power plants and retrofitting existing power plants to increase the efficiency of a working fluid power cycle.